A phase retarder is a type of optical element changing the polarization state of light passing through the same, and equally said a wave plate. When a light passes through an electromagnetic phase retarder, the polarization direction (direction of electric field vector) becomes a sum of two elements (an ordinary ray and an extraordinary ray) parallel or perpendicular to the optic axis, and changes after passing the phase retarder because the vector sum of two elements varies according to the birefringence and the thickness of the phase retarder. At this time, the plate that changes the polarization direction of light 90 degrees is called a quarter-wave plate (λ/4) and the plate that changes the polarization direction of light 180 degrees is called a half-wave plate (λ/2).
At this time, the phase difference value of the phase retarder depends of the wavelength, the wavelength dispersion of the phase difference value is classified into normal wavelength dispersion, flat wavelength dispersion, and reverse wavelength dispersion.
The phase retarder showing the reverse wavelength dispersion is most useful among them because it has specific phase differences (λ/4, λ/2, and so on) in wide wavelength band but the phase retarders formed from common resin films show normal wavelength dispersion in general.
In order to resolve such problem, many studies are being carried out. For example, Japanese Patent Publication Nos. 1998-068816, 1998-090521, 1999-052131, and 2000-002841 disclose laminate-type phase retarders formed by laminating a plurality of optically anisotropic layers. However, the laminate-type phase retarder having a lamination structure of a plurality of optically anisotropic layers has a disadvantage of low production yield and high production cost because the production process of the same needs a complicated process of not only arranging a plurality of films but also controlling the optical orientation of the films.
Meanwhile, a method of preparing a broadband λ/4 wave plate including only one phase retarder by inducing reverse dispersion through a film drawing has been already noticed. However, such wave plate is unsuitable for liquid crystal display devices requiring lamellation because the thickness of the plate is 100 μm or more.
And, Japanese Patent Publication No. 2002-267838 discloses a rod-type liquid crystal compound and a method of using a liquid crystal composition including a non-liquid crystal material that is oriented vertically to the major axis of said compound, for the purpose of preparing a thin layer broadband wave plate. However, in the case of the composition, there is a disadvantage that the reverse wavelength dispersion cannot be induced when the mixing ratio of the non-liquid crystal material is low and the liquid crystalline characteristics of the composition itself may be lost when the mixing ratio is high.
Therefore, the development of a thin broadband phase retarder which can exhibit stable reverse wavelength dispersion is required and particularly the study for the liquid crystal compound that makes it possible to prepare the phase retarder by a more simplified method is urgent.